© 1974 by The Society for Integrative and Comparative Biology
Functional Organization of the Neural Control of Circulation in Aplysia
Department of Physiology, University of California San Francisco, San Francisco, California 94143
Department of Neurobiology and Behavior, Public Health Research Institute of the City of New York New York, New York 10016
The abdominal ganglion of Aplysia provides a useful model for studying the functional organization of motor systems. Here we review studies of the neural network controlling circulation, emphasizing the organizational features it may share with other motor systems controlled by the abdominal ganglion. We identified seven motor neurons to the heart and vascular system. Motor neurons having similar motor effects (e.g. the two heart inhibitors, or the three vasoconstrictors), together with cells of unknown motor function located near them, make up distinct homogeneous cell groups. The members of each group appear to be nearly identical with respect to biophysical and neurochemical properties, size and effectiveness of synaplie inputs, and firing patterns. There are no interconnections between the members of the groups, but five interneurons innervate the homogeneous groups in various combinations, exciting some groups and inhibiting others. Two of the interneurons, Interneuron I (cell I10) and Interneuron II, are command cells which produce centrally generated motor programs in the absence of sensory feedback. Eacli command apparently codes for a specific homeostatic function, such as increased cardiac output. Coordination of the two commands is achieved by mutual inhibitory connections between them, ensuring that the motor neurons of the system receive only one command at a time. Some synaptic connections made by the command interneurons appear to be functionally ineffective; the possible significance of them is discussed. Available evidence suggests that many features of the network controlling circulation may be characteristic of other visceromotor systems of the abdominal ganglion.